This invention relates to an optical system having a nonspherical dome window, and, more particularly, to the baffling of stray light to prevent its entry into the detector system and its sensor.
An optical system includes an optical train with a sensor that receives radiated energy from a scene and converts it to an electrical signal. The electrical signal is provided to a display or further processed for automated pattern recognition. The sensor is fragile and is easily damaged by dirt, erosion, chemicals, or high air velocity.
In service, the sensor is placed behind a transparent, dome-shaped window through which it views the scene and which protects the sensor from such external effects. If the dome-shaped window is nonspherical, highly curved, and thick, it introduces significant wavefront aberration into the optical rays that pass through it on the way to the sensor. As discussed in U.S. Pat. No. 6,028,712, a transparent optical corrector may be placed in the optical path between the dome and the sensor to compensate for the aberration introduced by the nonspherical window.
Reflections from one or more thicknesses of transparent material may introduce stray light rays into the optical system that are unrelated to the scene light rays that are the subject of interest. An analogy, although somewhat imperfect, is the pattern that may sometimes be seen as the reflection from the windshield by the driver of an automobile. Under the right light conditions, the driver may see reflections in the windshield of objects outside the automobile that are not in the viewed scene. The pattern recognition system of the human mind can normally distinguish the viewed scene from the reflected pattern, but the pattern recognition systems of presently available image processors are not that sophisticated.
In the optical system, the stray light rays, if reflected into the sensor, may be misinterpreted by the pattern recognition system as having come from the scene, may obscure the scene, or may blind the sensor if sufficiently strong. One particularly troublesome source of stray light rays is the sun. Even after the light rays of the sun are reflected multiple times, they may still be orders of magnitude brighter than objects of interest in the scene.
There is a need for an approach to preventing stray light from interfering with the sensing of a scene in an optical system. The present invention fulfills this need, and further provides related advantages.
The present invention provides an optical system having a conformal (nonspherical) outer dome and an optical corrector. Stray light is excluded so that it cannot reach the sensor and damage the sensor and/or be misinterpreted in the pattern recognition process. The present approach does not require any modification of the detector system and greatly reduces the amount of computation required to distinguish stray signals. It is readily implemented in such optical systems.
In accordance with the invention, an optical system comprises a nonspherical outer dome that is rotationally symmetric about a central axis, a detector system including a sensor, and an optical corrector comprising a transparent body having an optical corrector shape responsive to a shape of the outer dome and positioned in an optical path between the outer dome and the detector system. There is additionally at least one light baffle positioned in the optical path between the outer dome and the detector system and fixed in space relative to the central axis. Each light baffle comprises a frustoconical tube (i.e., a tubular wall with a hollow interior) that is rotationally symmetric about the central axis. Where there are multiple baffles, they are desirably shaped so that the extrapolated apex of the innermost baffle is farther from the nose of the outer dome than the extrapolated apex of the outermost baffle.
There may be a single baffle, two baffles of increasing diametral size, three baffles of increasing diametral size, or more baffles if needed. The baffles are typically affixed to an inner surface of the outer dome or to a surface of the optical corrector.
An additional baffling effect may be achieved with a finned baffle having a set of fins supported on the baffle. Each fin extends radially outwardly from an outer surface of the frustoconical baffle and lies parallel to the central axis. The fins are symmetrically positioned about the central axis, typically with six fins in six-fold symmetry about the central axis. The finned baffle, if any, is preferably the innermost of the baffles if multiple baffles are present, but others of the baffles may be finned as well.
The innermost baffle desirably has a base diameter substantially equal to a diameter of an entrance pupil of the optical system and a length such that no interior reflected rays may pass through its center. Moving outwardly, the next baffle desirably has a base diameter such that a first skew ray may reflect off the outer dome and pass completely around the first baffle, and a length such that no interior reflected rays may pass through its center. These design principles are followed in selecting the positions and lengths of additional baffles.
The present approach recognizes that, once stray light has reached the detector system including its lens system and sensor, the stray light cannot be readily distinguished by the pattern recognition computer from a light ray of interest from the scene, and may even damage the sensor if sufficiently strong. A baffle system is therefore used to prevent stray light from reaching the detector system. However, the introduction of a baffle system may interfere with the transmission of light reaching the sensor, and may also have its own thermal signature that is sensed by the sensor. The present thin-ring frustoconical baffles minimize these potential adverse effects while preventing a large fraction of possible types of stray light from reaching the sensor. The use of this physical baffling to prevent stray light from reaching the sensor avoids the need for using large portions of the computing power of the pattern recognition processor for negating spurious signals. The risks of stray light obscuring the scene or blinding of the sensor by stray light are also minimized.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.